Dan,
do not waist time replacing the regulator.
37 @ idle with the vacume on it & 44 with vacume off is perfect (forget about Hayens).
According to Jerry Lemond and his Isuzu Factory Repair Manual that's what it should be (if any thing it's even a little higher).

I went through a lot of grief after bumping my fuel pressure up (did you know you can adjust it on ours?).

Normal pressure should be 34-35 PSI with vacume on & 44-45 PSI with vacume off.

My suggestion is change the fuel filter & check to see how's the air filter.

If it dosn't help let me know & i'll tell you what to check next (but it should do the trick <img src="/forums/images/graemlins/smile.gif" alt="" />).

Just my $.02